The average 12bore rifle with 2.5"chamber(65mm) was down in the puny end of the borerifles. Most of the hunting litterature mention 12bore as a "5 dramer". I know other 12s were made, but no-one mentions it in the huntinglitterature in the times of "borerifles". Some hunters of the "borerifle-period" had stated the 12bore as neither fowl or fish for hunting. The .577EX were much more preferred and more versatile because of power and trajectory and a more practical size doublegun.
Why do you seek info on 12s? Do you have one?